1. Field of the Invention
This invention relates to a method for a regeneration system for ion exchange beds used in deionization or demineralization of rinses such as circuit board rinses, aqueous cleaner rinses, plating/anodizing rinses, and tap water deionization systems, and particularly to that allowing for minimizing wastewater discharged from regeneration systems using highly simplified flow.
2. Background of the Art
Ion exchange technology has long been employed for the effective removal of objectionable ions from solutions. Applications include exchange of hardness for sodium (softening), of bicarbonates for chlorides (dealkalizing), and of cations and anions for hydrogen and hydroxyl ions (demineralization).
Ion exchange technology does facilitate a moderate reduction of volume by concentrating the impurities to be removed from water. Regeneration with brine, acid and/or caustic at 5-10% concentrations can produce spent regenerants containing percent levels of the impurity. This is of particular interest when using ion exchange to remove heavy metal contaminants from metal plating rinse streams. Upon regeneration, the contaminant ions, such as copper and nickel, are contained in percent concentrations instead of ppm concentrations and with a corresponding reduction in volume.
However, chemical regeneration increases the total solids of the waste chemicals to ultimately be disposed of. In fact, the total solids can be increased by a factor of three or more unless controls are employed to reduce the waste chemical volume containing the objectionable ions. In addition, the liquid volume of regenerant waste can exceed 15 Bed Volumes (BV) following normal backwash, chemical draw and rinsing steps.
In conventional regeneration of ion exchanges, the bed is first backwashed to loosen dirt and debris and redistribute the resins for better flow without channeling. Backwash flows for cation exchangers are typically 6 gpm per ft.sup.2 of bed area. For typical three-foot bed depth recommended by manufacturers, this amounts to 2 gpm/ft.sup.3. A backwash conducted for 20 minutes would therefore produce 40 gallons of waste water per cubic foot of resin. For anion exchangers, the flow is less because of the lower density. Nonetheless, approximately 15 gallons of waste are spent per cubic foot of resin. Chemical draw, the next step in regeneration is typically carried out at concentrations of 4-6% or approximately 0.50 lbs (active) per gallon. With regenerant levels of 6-8 lbs/ft.sup.3, an average of 15 gallons of waste is produced. The next step is the displacement rinses to propel the regenerant through the beds. These rinse volumes are typically 2-3 BV or 15-20 gallons/ft.sup.3. Following the displacement rinse, a rapid rinse is employed at full flow to purge the residual regenerants from the system and prepare the bed for next cycle. This is referred to as the quality rinse and can average 30 minutes for each resin at 2 gpm/ft.sup.3. Hence the regeneration waste total for a conventional system typically add up to 18 BV for a cation, and 14.7 BV for an anion (1 Bed Volume is the interior volume of the bed, occupied by the resin and the remaining void in the bed; in this case, 7.5 gals or 1 ft.sup.3 of liquid).
Prior attempts to reduce waste volume have employed reuse of regenerants. U.S. Pat. No. 5,352,345 to Byszcwski et al discloses a method in which exhausted regenerating solutions from either a cationic or anionic exchange column are converted into fresh regenerating solutions by using an electrodialytic water splitter, an acid or base purification unit, or any combination thereof However, use of the special device, i.e., the electrodialytic water splitter, the acid or base purification unit, or any combination thereof, is very costly, and the piping system is complicated.
U.S. Pat. No. 4,652,352 to Carl J. Saieva discloses a process for recovering metals from dilute solution utilizing ion exchange in combination with ammonium salt regeneration solutions. However, use of the special device, i.e., the electrolytic recovery system is very costly. Further, the system has serious drawbacks, i.e., simple removal of the metals cannot render the water reusable because the solution contains metal salts such as copper chloride and the other half of the metal salts after removal of the metals will continue to accumulate. In addition, there is no teaching of minimizing rinse.
In addition, Rohm & Haas Amber Hi-Liters No. 120 describes reuse of regenerant. As described, the first third of the regenerant will be overly diluted by the existing water in the resin column and the water in the void volumes between the resin beads. This is sent to waste. The second 1/3, being the most spent, would also be sent to waste. This final 1/3 is suggested for reuse as the first third of the subsequent chemical draw cycles. Thus, the reuse of regenerant is limited to recycling only 1/3 of the entire chemical draw cycle.